The book was Wentworth’s Algebra—­elementary principles.  Painful recollections of my boyhood and the binomial theorem rose in my mind as I let the leaves turn under my fingers.  “What do you make of it?” I asked.

His tone became even more confidential.  “Part of it, monsieur, is in English; that is plain.  I have found an English word in it that I know—­ the word ‘O.’  But much of the printing is also in Arabic.”

“Arabic!” I exclaimed.

“Yes, monsieur, look there.”  He laid a fat forefinger on “(a + b)2 = a2 + 2ab + b2.”  “That is Arabic.  Old Gaston has been to Algeria, and he says that he knows Arabic as well as he does French.  He looked at the book and told me it was Arabic.  Truly!  Truly!”

“Did he translate any of it for you?”

“No, monsieur; his eyes pained him this afternoon.  He says he will read it to-morrow.”

“But you must return the book to-night.”

“That is true.  Eh!  It leaves the mystery deeper than ever, unless monsieur can find some clue in those parts of the book that are English.”

I shed no light upon him.  The book had been Greek to me in my tender years; it was a pleasure now to leave a fellow-being under the impression that it was Arabic.

But the volume took its little revenge upon me, for it increased my curiosity about Professor Keredec and “that other monsieur.”  Why were two grown men—­one an eminent psychologist and the other a gray-haired youth with a singular air—­carrying about on their walks a text-book for the instruction of boys of thirteen or fourteen?

The next day that curiosity of mine was piqued in earnest.  It rained and I did not leave the inn, but sat under the great archway and took notes in colour of the shining road, bright drenched fields, and dripping sky.  My back was toward the courtyard, that is, “three-quarters” to it, and about noon I became distracted from my work by a strong self-consciousness which came upon me without any visible or audible cause.  Obeying an impulse, I swung round on my camp-stool and looked up directly at the gallery window of the salon of the “Grande Suite.”

A man with a great white beard was standing at the window, half hidden by the curtain, watching me intently.

He perceived that I saw him and dropped the curtain immediately, a speck of colour in his buttonhole catching my eye as it fell.

The spy was Professor Keredec.

But why should he study me so slyly and yet so obviously?  I had no intention of intruding upon him.  Nor was I a psychological “specimen,” though I began to suspect that “that other monsieur” Was.

CHAPTER V